The present invention relates generally to gas turbine engines and more particularly to an improved hollow fan blade for a gas turbine engine.
A gas turbine engine, such as a turbo fan engine for an aircraft, includes a fan section, a compression section, a combustion section and a turbine section. An axis of the engine is centrally disposed within the engine and extends longitudinally through the sections. The primary flow path for working medium gases extends axially through the sections of the engine. A secondary flow path for working medium gases extends parallel to and radially outward of the primary flow path.
The fan section includes a rotor assembly and a stator assembly. The rotor assembly of the fan includes a rotor disc and plurality of radially extending fan blades. The fan blades extend through the flow path and interact with the working medium gases and transfer energy between the fan blades and working medium gases. The stator assembly includes a fan case, which circumscribes the rotor assembly in close proximity to the tips of the fan blades.
During operation, the fan draws the working medium gases, more particularly air, into the engine. The fan raises the pressure of the air drawn along the secondary flow path, thus producing useful thrust. The air drawn along the primary flow path into the compressor section is compressed. The compressed air is channeled to the combustion section where fuel is added to the compressed air and the air/fuel mixture is burned. The products of combustion are discharged to the turbine section. The turbine section extracts work from these products to power the fan and compressed air. Any energy from the products of combustion not needed to drive the fan and compressor contributes to useful thrust.
In order to reduce weight, the fan blades in some gas turbine engines are hollow. Each fan blade is made by combining two separate detail halves. Each half includes a plurality of cavities and ribs machined out to reduce the weight while forming a structurally sound internal configuration. These halves are subsequently bonded to form the hollow fan blade. The hollow fan blade is then subjected to forming operations at extremely high temperatures at which time it is given an airfoil shape and geometry. During the forming operation, the two detail halves are twisted and cambered under high temperatures to the desired shape. Inherent to the hollow fan blade design is a set of “skins” on the convex and concave side of the airfoil. These skins undergo significant compressive loading during the bonding and forming operations. At elevated temperatures, these skins do no possess the robustness to withstand this loading, and deform by sagging or drooping inward toward the center of the blade. To prevent collapse of the cavities during the forming process, the cavities are filled with high-pressure gas to maintain their geometry during the forming operation.
To a large extent, the internal geometry of the hollow fan blades has been designed to provide bird-impact capabilities. The previous hollow fan blades had an internal geometry comprising numerous machined internal cavities and associated ribs primarily running radially with secondary ribs running chord-wise.
There are several drawbacks to the known hollow fan blades. First, using the high-pressure gas required during forming operation increases time and cost of the operation. Additionally, the intersecting ribs in the hollow fan blades require numerous different diameter cutters and numerous cutting operations to achieve the small fillets that the objectives dictate. This also increases the time and cost of manufacturing the hollow fan blades.